The present invention relates to devices for generating and manipulating coherent matter waves, in particular to an atom laser, as well as to a method of generating coherent matter waves in a pulsed or continuous mode.
The generation of coherent matter waves generally bases on the extracting or decoupling of atoms or atom groups from so-called Bose-Einstein-condensates (in the following: BEC""s). In a BEC, a macroscopic number of bosonic atoms occupy the ground state of the system, which can be described by a single wave function. Although the existence of BEC""s has been predicted by Albert Einstein and Satyendra Nath Bose even in 1924, the first experimental demonstrations of BEC""s in alkali-metal gases have been obtained in 1995 only (see M. H. Anderson et al. in xe2x80x9cSciencexe2x80x9d, Vol. 269, 1995, p. 198; C. C. Bradley et al in xe2x80x9cPhys. Rev. Lett.xe2x80x9d, Vol. 75, 1995, p. 1687, Vol. 78, 1997, p. 985; and K. B. Davis et al in xe2x80x9cPhys. Rev. Lett.xe2x80x9d, Vol. 75, 1995, p. 3969). The BEC generation bases on the trapping of gas atoms in a magneto-optical trap (MOT) and a magnetic trap and cooling the trapped atoms to temperatures of less than one microkelvin.
The simplest way to magnetically trap atoms is to use the quadrupole field created by two coils with currents in opposite directions. In this configuration, the atoms can easily be loaded from a MOT (see C. Monoroe et al. in xe2x80x9cPhys. Rev. Lett.xe2x80x9d, Vol. 65, 1990, p. 1571) into the magnetic trap since both traps have a common center and share the same symmetry. The magnetic trapping potential xcexc|B(r)| is given by the spatially varying magnetic field B(r) and the effective magnetic moment xcexc of the atom. For an atom in a weak-field-seeking state, the potential in a quadrupole trap grows linearly with distance from the trap center, where the magnetic field is zero. This zero field in the trap center represents the major disadvantage of quadrupole traps. The cold atoms can be removed from the trap center due to nonadiabatic spin flips (see e.g. W. Petrich et al. in xe2x80x9cPhys. Rev. Lett.xe2x80x9d, Vol. 74, 1995, p. 3352). Due to this disadvantage, it is very difficult to achieve or manipulate a BEC in a quadrupole trap.
The above disadvantage of magnetic quadrupole traps has been solved with time-averaged orbiting potential (TOP) traps and with Ioffe traps (see the above publications of C. C. Bradley et al. and W. Petrich et al., and M.-O. Mewes et al. in xe2x80x9cPhys. Rev. Lett.xe2x80x9d, Vol. 77, 1996, p. 416). The TOP trap bases on the superposition of a quadrupole field and a rotating magnetic field. With this technique, a low potential is obtained in the trap center, which avoids spin flips in the center of the trap. TOP traps are not convenient for manipulating BEC""s in an effective way as only a small number of atoms can be trapped. Furthermore, the rotating magnetic field vector interferes with additional manipulating fields e.g. for the extraction of atoms from the BEC.
In a magnetic Ioffe trap, a two-dimensional quadrupole field is formed which extends in the third dimension as a cigar-shaped channel in which the atoms are trapped. Ioffe traps suffer from a difficulty in aligning the center of the magneto-optical traps for collecting and pre-cooling atoms with the center of the magnetic trap. Furthermore, Ioffe traps typically dissipate several kilowatts of power, which causes considerable cooling, stabilization and switching problems. The generation of coherent matter waves bases on the extraction of atoms from a BEC. The atoms which are in a magnetically trapped state are transferred into an untrapped state. The transition is induced by an external radio frequency (rf) field. A first output coupler for said extracted matter waves from a BEC has been described by M.-O. Mewes et al. in xe2x80x9cPhys. Rev. Lett.xe2x80x9d, vol. 78, 1997, p. 582. In this experiment, the transition from the trapped to the untrapped state has been caused by a short rf pulse with a duration in the range of about one xcexcs. This first matter wave generator or atom laser was restricted to a pulsed output, i.e. it was capable to generate only single coherent atom groups but not a continuous atom wave.
Additionally, the pulsed extraction has the following drawback. A fraction of the entire condensate is transferred quasi simultaneously from the trapped state into the untrapped state. In the untrapped state, the escaping atoms are accelerated by the gravitation potential, in which they occupy energy levels (eigenvalues) which are distributed over a certain energy band. This energy band is relatively broad as the coupling rate from the trapped state to the untrapped state is high. Due to the broad energy band, the extracted atoms have a broad velocity distribution reducing the stability and collimation of the extracted matter wave.
Finally, the conventional magnetic traps are characterized by field fluctuations so that they are not capable to a allow a stable outcoupling of atoms from the trap in a precise and reproducible manner.
The object of the invention is to provide an improved magnetic trap with an increased stability of the trap potential and an improved generator of continuous or quasicontinuous coherent matter waves, which particularly are characterized by a narrow energy band width and high beam collimation. A further object of the invention is to provide a device for manipulating coherent matter waves. Yet another object of the invention is to provide a method of generating coherent matter waves.
Generally, a matter wave generator is described which comprises an atom laser device for the creation of coherent matter waves by extracting atoms from a magnetic trap and a magnetic field shielding device surrounding the magnetic trap. Furthermore, a matter wave manipulator is described which comprises at least one magnetic trap, which is shielded against external magnetic fields with a magnetic field shielding device surrounding the magnetic trap. The magnetic field shielding device is preferably a housing made of a magnetic shielding material (material with a high magnetic permeability).
According to a first aspect of the invention, a new magnetic trap configuration for an atom laser is described which incorporates both a quadrupole and a loffe configuration (in the following: QUIC trap), wherein the magnetic trap is provided with the shielding device protecting the trap against external magnetic fields. As the shielding device, generally an active shielding technique can be used as it is known as such from electron microscopes. Active shielding techniques are based on the measurement of external magnetic fields with magnet sensors and controlling compensation coils in dependence on the sensor signals. Preferably, the housing made of magnetic shielding material is used as the shielding device. The QUIC trap comprises a plurality of trap coils which are capable of forming a common trap with a variable trap shape. The trap shape can be changed from a quadrupole shape which facilitates the atom supply to the QUIC trap, to a loffe shape allowing a stable formation of a BEC from the supplied atoms. According to their function, two of the trap coils are called quadrupole coils which form the quadrupole trap as it is known from conventional quadrupole traps as such. A third trap coil is called a Ioffe coil as this third coil has the function to form, in co-operation with the quadrupole coils, the Ioffe trap shape. During the formation of the Ioffe shape, the trap center moves from the center of the quadrupole coils towards the Ioffe coil so that the trapped atoms are shifted within the coil arrangement.
Preferably, the quadrupole coils are identical coils being arranged along a common reference line with a quadrupole coil space therebetween. The third coil, the Ioffe coil, has a smaller dimension than the quadrupole coils and is positioned at least partially in the quadrupole coil space.
According to a further preferred embodiment of the invention, the trap coils are mounted on copper tubes which are connected with a cooling system and which are provided with means for avoiding eddy currents. As an additional measure against the generation of eddy currents, shielding plates are provided between the trap coils and the supporting copper tubes. Preferably, these shielding plates are made from the same material like the above shielding housing.
According to a second aspect of the invention, a matter wave generator or atom laser is described which comprises two magnetic traps a first of which is a magneto-optical trap for trapping and precooling gas atoms and the second of which is being capable of forming magnetic trap with the above new configuration. The second magnetic trap can be operated in two different modes. In the first (magneto-optical) mode, a cold gas is prepared by laser cooling. In the second (magnetic) mode, atoms are further cooled by evaporative cooling to form a BEC and finally extracted from the BEC. To this end, the matter wave generator is further provided with a cooling and outcoupling coil which has a double function. Firstly, the cooling and outcoupling coil is used for the evaporative cooling of the gas to reach the BEC state. Secondly, this coil induces transitions from trapped to untrapped atom states.
According to a preferred embodiment of the matter wave generator, the second magnetic trap is provided with a shielding device protecting the magnetic trap against external magnetic fields.
According to a third aspect of the invention, a matter wave manipulator is described which comprises at least one magnetic trap (magnetic manipulator trap) capable of forming a potential for subjecting a matter wave to a change of the traveling direction thereof. Furthermore, the matter wave manipulator contains a switching device adapted to induce transitions from a non-magnetic state to a magnetic state and vice versa in the atoms of the matter wave. The nonmagnetic state corresponds to the above untrapped state, in which the atoms have practically no interactions with the magnetic potential of the magnetic trap. In the magnetic state corresponding to the above trapped state, the atoms interact with the magnetic potential. The switching device comprises a laser source in a first embodiment. Alternatively, it is implemented with a rf emitter like the above cooling and outcoupling coil.
According to a preferred embodiment of the matter wave manipulator, at least the magnetic trap is provided with a shielding device protecting it against external magnetic fields. This shielding device can be the same shielding as the magnetic trap of-the atom laser described above.
The matter wave manipulator can have a wave directing element or a beam forming element, like a mirror or a lens. The function of the matter wave manipulator is selected via the definition of a predetermined potential in the magnetic manipulator trap.
According to a fourth aspect of the invention, a method of generating coherent matter waves using a magnetic trap with the above new configuration is described. Essential steps of this method are the transfer of cooled atoms from a quadrupole trap to a Ioffe trap with the above QUIC trap as well as the extraction of atoms from the Ioffe trap using a cooling and outcoupling coil.
Compared with the conventional trap configurations and attempts to generate matter waves, the invention has the following essential advantages. By spatially separating the centers of the quadrupole and the Ioffe geometry in the QUIC trap, a magnetic trap of unexpected simplicity and efficiency is created. The QUIC trap consists in a preferred embodiment of merely three coils and dissipates no more than 600 W while operating at a current of only 25 A. The QUIC trap provides an extremely stable trapping potential allowing for the first time a continuous wave output coupling as the magnetic field fluctuations experienced by the trapped atoms are minimized. The level of fluctuations in the magnetic field is much less than the change of the magnetic field over the spatial sizes of the BEC. The compactness of the QUIC trap allows it to be placed inside a magnetic shielding housing which reduces the magnetic field of the environment and its fluctuations by a factor of approximately 100.
Further advantages are related to the operation of a matter wave generator and/or manipulator with the new magnetic trap. For the first time, a brightness can be reached which is up to ten orders of magnitude higher than the brightness of atom groups generated by conventional atomic beam sources.